Acousto-optic tunable filter

Abstract

An improved acousto-optic tunable filter infrared analyzer system useable in a variety of industrial and commercial combustion control applications includes an acousto-optic tunable filter having a crystalline material selected from the group consisting of mercurous chloride, mercurous bromide and mercurous iodide. Selectively polarized infrared radiation is passed at a predetermined plane of incidence to the crystal axis, which plane contains both the (110) and (001) axes. The system relies upon a narrow band pass tunable acousto-optic filter which is selectively tuned by predetermined rf frequency signals to selectively transmit the narrow band pass of interest which corresponds to a specific molecular species for identification and analysis. The system includes a microcomputer and associated memory function to measure and compare detected signals from an infrared detector which converts the filtered infrared signal to an electrical signal. The memory provides control signals for the computer and for controlling the sequence and frequency of rf energy applied to tune the filter. In this way, the near to far range infrared can be analyzed for absorption bands corresponding to predetermined molecular species such as combustion product gases, and a feedback signal generated to control the combustion process.

What is claimed is: 
     
       1. In combination with an automated acousto-optic tunable filter infrared analyzer system comprising: (a) means for directing infrared radiation through a sample species to be analyzed, which species have predetermined infrared absorption characteristics;   (b) means for focusing the infrared radiation upon an acoustic-optic tunable filter;   (c) an acoustic-optic tunable filter comprising an input polarizer for selectively polarizing the infrared radiation, and an optically aligned acoustic-optic crystal through which the selectively polarized infrared radiation is passed at a predetermined plane of incidence to the crystal optic axis, an acoustic transducer means coupled to a variable frequency rf power source and to the acoustic-optic crystal to launch acoustic waves in the crystal to interact with a selected narrow bandwidth portion of the polarized infrared radiation to make it distinguishable from the remaining infrared radiation, which selected narrow bandwidth portion is a function of the frequency of the rf energy and acoustic waves;   (d) an rf energy source connected to the acoustic transducer of the acousto-optic tunable filter;   (e) infrared radiation detection means which detects the output filtered infrared radiation from the filter and generates an output electrical signal as a function of the output filtered infrared radiation;   (f) computing means to which the detection means output electrical signal is applied for determining the species present in the sample cell, and including means for selectively actuating the rf energy source to determine the timing and frequency of rf energy applied to the acoustic transducer mated to the acousto-optic crystal to determine the infrared wavelength selectivity or tuning of the acousto-optic tunable filter; the improvement wherein said acousto-optic crystal is a selected mercurous halide crystal and the plane of incidence to the crystal axis is that plane containing both the (110) and (001) axes, and wherein the acoustic waves launched into said mercurous halide crystal are in the plane of said crystal containing both the (110) and (001) axes, and wherein the infrared radiation propagates through the mercurous halide crystal at an angle equal to approximately ten degrees to the (001) crystallographic axis and wherein said acousto-optic crystal has an input face which is cut so as to be normal to the incident infrared beam which beam output is diffracted at an angle of about six degrees to the incident beam and wherein said acousto-optic crystal has an exit optical face which is cut so as to be normal to the diffracted output beam.     
     
     
       2. The improved analyzer system set forth in claim 1, wherein the means for directing infrared radiation through a sample cell comprises a collimating mirror, which means for focusing the collimated infrared radiation transmitted through the sample cell comprises a focusing mirror. 
     
     
       3. The improved analyzer system set forth in claim 1, wherein the acousto-optic tunable filter includes an output polarizer which is optically aligned with the input polarizer and the acousto-optic crystal, which output polarizer is oriented transversely to the input polarizer to only transmit a selected narrow bandwidth portion of the infrared radiation. 
     
     
       4. The improved analyzer system set forth in claim 1, wherein the infrared radiation detection means comprises an infrared sensitive pyroelectric detector. 
     
     
       5. The improved analyzer system set forth in claim 1, wherein computing means includes a microprocessor and memory means for comparing the detected signal to predetermined molecular species indicative signals stored in the memory means, and wherein the memory means provides a predetermined sequence of signals which are applied to the microprocessor to be applied to a frequency synthesizer to vary the frequency of the rf energy applied to the transducer means coupled to the acousto-optic crystal to vary the selection of the narrow bandwidth portion of the infrared radiation which is analyzed, and wherein predetermined frequencies corresponding to predetermined molecular sample species are applied. 
     
     
       6. The improved analyzer system set forth in claim 1, wherein the sample species are provided in a sample cell which is disposed between the means for directing the infrared radiation and the means for focusing the infrared radiation upon the acousto-optic tunable filter. 
     
     
       7. The improved analyzer system set forth in claim 1, wherein the rf energy source includes an rf energy frequency synthesizer coupled by electronic signal gate means amplifier which is connected to the acoustic transducer, and the computing means includes a microprocessor and memory means for applying sequential pulsed control signals to the rf frequency synthesizer to predeterminedly vary the frequency of the rf energy applied to the acousto-optic tunable filter, and wherein control signals are applied to the electronic signal gate means to provide pulse width modulated rf energy to the rf amplifier. 
     
     
       8. The improved analyzer system set forth in claim 1 wherein the mercurous halide crystal is selected from the group consisting of mercurous chloride, mercurous bromide and mercurous iodide. 
     
     
       9. An acoustic-optic tunable filter comprising: an input polarizer for selectively polarizing infrared radiation, an optically aligned mercurous chloride crystal acousto-optic tunable filter through which the selectively polarized infrared radiation is passed at a plane of incidence containing both the (110) and (001) crystal axes;   an acoustic transducer means coupled to a variable frequency rf energy source and to said mercurous chloride crystal to launch acoustic waves into said crystal in the plane of said crystal containing both the (110) and (001) axes and wherein the infrared radiation propagates through the mercurous chloride crystal at an angle equal to approximately ten degrees to the (001) crystallographic axis in order, said acoustic waves interacting with a selected narrow bandwidth portion of the polarized infrared radiation in order to make it distinguishable from the remaining infrared radiation, which selected narrow bandwidth portion if a function of the frequency of the rf energy and the acoustic waves.   
     
     
       10. An acousto-optic tunable filter comprising: an input polarizer for selectively polarizing infrared radiation, an optically aligned acousto-optic tunable filter of a crystalline material selected from the group consisting of mercurous chloride, mercurous bromide and mercurous iodide through which the selectively polarized infrared radiation is passed at a plane of incidence containing both the (110) and (001) crystal axes; and an acoustic transducer means coupled to a variable frequency rf energy source and to said AOTF crystal to launch acoustic waves into said crystal in the plane of said crystal containing both the (110) and (001) axes in order to interact with a selected narrow bandwidth portion of the polarized infrared radiation which propagates through said crystal at an angle equal to approximately ten degrees to the (001) crystallographic axis, in order to diffract the selected narrow bandwidth portion and make it distinguishable from the remaining infrared radiation, which selected narrow bandwidth portion is a function of the frequency of the rf energy and the acoustic waves said crystal having an optical input and optical output face, which input face is normal to the polarized infrared radiation and which output face is normal to the diffracted infrared radiation.